1. Field of the Invention
The present invention relates to a method of forming a metal contact opening and, more particularly, to a method of forming a metal contact opening with a width that is smaller than the minimum feature size of a photolithographically-defined opening.
2. Description of the Related Art
A metal oxide semiconductor (MOS) transistor is a well-known semiconductor device which can be implemented as either an n-channel (NMOS) device or a p-channel (PMOS) device. A MOS transistor has spaced-apart source and drain regions, which are separated by a channel, and a gate that lies over the channel. The gate is insulated from the channel by a gate dielectric layer. A metal-gate MOS transistor is a type of MOS transistor that utilizes a metal gate and a high-k gate dielectric layer.
Metal-gate MOS transistors are connected to a metal interconnect structure that electrically connects the MOS transistors together to form an electrical circuit. The metal interconnect structure includes layers of metal traces that are electrically isolated from each other by layers of isolation material, and metal vias that extend through the layers of isolation material to electrically connect adjacent layers of metal traces.
The metal interconnect structure also includes metal contacts that extend through the bottom layer of isolation material to make electrical connections to the source and drain regions of the MOS transistors. The metal contacts are formed in metal contact openings that extend through the bottom layer of isolation material to expose the source and drain regions.
Conventionally, the metal contact openings are fabricated by forming a pattered photoresist layer on the bottom isolation layer, which touches and lies over the source and drain regions. Once the patterned photoresist layer has been formed, the bottom isolation layer is etched until the source and drain regions have been exposed.
The etch forms source metal contact openings that expose the source regions, and drain metal contact openings that expose the drain regions. The patterned photoresist layer is then removed. After this, silicide layers are formed on the source and drain regions, followed by the formation of metal contacts that lie in the source and drain metal contact openings, and touch the source and drain silicide layers and the bottom isolation layer.
Thus, in the conventional approach, the widths of the source and drain metal contact openings are determined by the widths of the openings in the patterned photoresist layer. As a result, the minimum widths of the source and drain metal contact openings are determined by the minimum feature size that can be photolithographically printed with adequate control.
The minimum feature size has two basic limits: the smallest image that can be projected onto a wafer, and the resolving capability of the photoresist to make use of that image. The smallest image that can be projected onto a wafer is determined by the wavelength of the imaging light and the numerical aperture of the projection lens. The resolving capability of the photoresist is determined, in part, by the shape of the image projected onto the wafer.
For example, when long parallel lines are projected onto the wafer, the photoresist has a higher resolving capability along the lengthwise edges of the lines than when square or circular shapes are projected onto the wafer. As a result, long parallel lines can be formed with smaller minimum feature sizes than square or circular openings.
To increase the density of devices formed on the wafer and thereby reduce costs, the minimum feature size has been continuously scaled down, primarily by decreasing the wavelength of the imaging light and increasing the numerical aperture. However, the density of devices formed on the wafer can be further increased if the metal contact openings could be formed to have widths that are smaller than the minimum feature size of a photolithographically-defined opening. Thus, there is a need for a method of forming a metal contact opening with a width that is smaller than the minimum feature size of a photolithographically-defined opening.